SOURCES OF THE ESCUINTLA AND LA DEMOCRACIA DEBRIS AVALANCHES, GUATEMALA

Two large debris avalanche deposits occur south of the Fuego Volcanic Complex on the coastal plain of Guatemala. The larger deposit, known as the Escuintla Debris Avalanche (EDA), is 27 km long by 18 km wide, and has an estimated volume of 9 km³. The La Democracia Debris Avalanche (LDDA) is 15 km long, 10 km wide, and about 2.4 km³ in volume. Previous studies have postulated that the source of the EDA was Meseta volcano, the northernmost vent of the Fuego Volcanic Complex. A steep east-facing scarp on Meseta presumably formed by sector collapse, generating the EDA. The scarp exposes a thick stratigraphic section of lavas and tephras that represents a significant portion of Meseta’s eruptive history. The LDDA, has been tentatively linked to either the Fuego or Acatenango volcanic complexes, although no obvious collapse scar has been identified. In order to identify the source volcano for each deposit, lava block samples collected from each deposit were compared to lavas sampled at Meseta volcano. Specifically, a suite of 27 consecutive lavas collected in the Meseta escarpment were compared petrographically and geochemically to 24 samples collected during a longitudinal traverse of the EDA, and 10 samples from the LDDA. The Meseta samples are mostly basaltic andesites and a few andesites, containing plagioclase, pyroxenes, and olivine. Samples collected from the EDA have a broader compositional range and are predominantly andesites and dacites. In addition, they have less olivine, and a few samples contain amphibole, quartz, and K-feldspar. On most geochemical plots the EDA samples form a distinct group, offset from the Meseta samples. Collectively, this data strongly suggests that the EDA did not originate from the scarp on Meseta volcano as previously proposed. Geochemical comparisons to other nearby volcanoes (Agua and Acatenango) will be made to assess which volcano collapsed to from the EDA. Samples from the much smaller LDDA suite are generally similar mineralogically and geochemically to the Meseta samples. Therefore, Meseta cannot be ruled out as a possible source for the LDDA, although its location precludes origin from the Meseta scarp. Data from Acatenango and Fuego volcanoes will be also be assessed to establish the most likely source for the LDDA.